Though there is no interaction between the 2 medications there is still an issue that you need to be aware of. Cephalexin is a semisynthetic cephalosporin antibiotic. Cephalosporins may be associated with a fall in prothrombin activity. Those at risk include patients with renal or hepatic impairment, or poor nutritional state, as well as patients receiving a protracted course of antimicrobial therapy, and patients previously stabilized on anticoagulant therapy. Prothrombin time should be monitored in patients at risk and exogenous vitamin K administered as indicated. Therefore assuming you were stabilized on coumadin, you are at risk and need to be monitored.

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